1. Field of the Invention
The present invention relates in general to the field of information processing, and more specifically to a multiple input-multiple output (“MIMO”) communication system and method for estimating a MIMO channel using transmission device and receiving device peak-limited pilot signals.
2. Description of the Related Art
The demand for wireless communication systems continues to expand. Wireless communication systems transmit and receive signals within a designated electromagnetic frequency spectrum. Wireless communication systems involve data communication between a subscriber station and a base station. Base stations and subscriber stations can be both transmitting devices and receiving devices when both base stations and subscriber stations are equipped with a receiver and a transmitter. Base stations generally communicate with multiple subscriber stations. Subscriber stations communicate directly with a base station and indirectly, via the base station, with other subscriber stations. The number of base stations depends in part on the geographic area to be served by the wireless communication system. Subscriber systems can be virtually any type of wireless one-way or two-way communication device such as cellular telephones, wireless equipped computer systems, and wireless personal digital assistants. The signals communicated between base stations and subscriber stations can include data such as voice, electronic mail, and video.
FIG. 1 depicts a multiple input, multiple output (MIMO) communication system 100 with a subscriber station 102 and a base station 104. The MIMO communication system 100 can include other subscriber stations and base stations. In a MIMO system, each subscriber station 102 includes a transmitter 105 with a respective array of antennas 106.1-106.k. Each base station 104 includes a receiver 107 with a respective array of antennas 108.1-108.m for receiving signals, where k and m are integers greater than one (1). The values of k and m are a matter of design choice, and k may or may not equal m. MIMO communication systems utilize multiple base station antennas and multiple subscriber station antennas to improve performance.
Subscriber station 102 includes precoder 110 to precode the signal to be transmitted, signal x(n), in accordance with a precoder matrix P. (Note, a vector is represented by bolded lower case letters, such as x, and a matrix is represented by bolded upper case letters, such as P). Signal x(n) is a k element vector, and represents the nth sample of a sequence of data x. During a complete transmission, generally each sample of sequence x is transmitted. In at least one embodiment, the precoder matrix P is a k by k matrix with each column of precoder matrix P representing complex beam forming weight vectors. The element values of precoder matrix P are a matter of design choice. Many conventional techniques exist to design precoder matrix P. The elements of precoder matrix P are generally designed so that the precoder matrix P precodes signal x(n) to allow the base station 104 to distinguish between the transmission signal of subscriber station 102 and other subscriber stations (not shown). The k element vector y represents the output of precoder 110 and, in at least one embodiment, is the product of precoder matrix P and signal x(n), i.e. y=Px(n).
A frequently encountered disadvantage of MIMO communication systems involves output samples with a high peak-to-average power ratio (PAR). Subscriber station 102 and base station 104 have a limited amount of power per antenna to transmit a signal, such as the transmission signal vector y(n)′. Typically, the maximum power available for transmission is correlated with a maximum amplitude signal sample. If the ratio between the maximum amplitude signal sample and the average amplitude of the signal samples is large, the amount of power allocated to transmit average power signal samples is relatively low. A high PAR often results in low power efficiency and possible non-linear distortion.
Various peak limiting techniques exist to decrease the PAR while attempting to minimize distortion and increase power efficiency. “Tone Reservation” represents one such peak limiting technique. An example of tone reservation to reduce the PAR is described in J. Tellado-Mourelo, Peak to Average Power Reduction for Multicarrier Modulation, Ph.D. dissertation, Stanford University, Stanford, Calif., September 1999 (referred to herein as Tellado), which is incorporated by reference herein in its entirety.
Peak limiters 112.1-112.k apply peak limiting technology, such as tone reservation, to peak limit the first through kth elements of the precoded signal y(n) and generate a peak limited transmission signal y(n)′ for each of the samples x(n) of pilot sequence x. Subscriber station 102 then transmits transmission signal y(n)′ to base station 104 and repeats the transmission for each of the transmission signal samples y(n)′. In at least one embodiment, peak limiting is performed serially on each of samples y(n) using a processor executing a peak limiting algorithm.
Subscriber station 102 transmits signal vector y(n)′ via a channel represented by the channel matrix H. The channel matrix H represents a channel gain between the antenna array 106.1-106.k and the antenna array 108.1-108.m Thus, the channel matrix H can be represented by a k×m matrix of complex coefficients. The coefficients of the channel matrix H depend, at least in part, on the geometry and material composition of signal reflective objects.
In a correlated communication system, the subscriber station 102 uses a pilot sequence vector xp to allow the receiving base station 104 to determine an estimate of the channel matrix H. The content of each sample xp(n) of the pilot sequence xp is a matter of design choice. In one embodiment, the pilot sequence xp is a constant amplitude, zero autocorrelation (CAZAC) sequence. The subscriber station transmits a precoded, peak limited pilot signal yp(n)′ to base station 104.
Base station 104 includes a receiver 107 that receives a signal r on antennas 108.1-108.m. The received signal r represents the peak limited pilot signal yp′ as modified by the channel matrix H and noise n, such that received signal r equals the product of the channel matrix H and the peak limited pilot signal yp(n)′ plus noise, i.e. r=Hyp(n)′+η. The noise vector η is an m element vector representing noise added by, for example, random vibration of electrons in each of antennas 108.1-108.m. Channel estimator 114 determines an estimated channel matrix Ĥ. Several conventional methods exist to determine the elements of the estimated channel matrix Ĥ using the known pilot sequence xp.
Once the estimated channel matrix {circle around (H)} is known, the base station 104 uses decoder 116 to decode future received signals r=Hyp(n)'+η. Since the channel matrix H can change over time and as the location of the subscriber station 102 changes, the process used to determine the estimated channel matrix {circle around (H)} can be repeated as desired. Additionally, the process used to determine the estimated channel matrix {circle around (H)} can be reversed with the base station 104 becoming the transmitting device and the subscriber station 102 becoming the receiving device.
Conventionally, since peak limiting relates to transmission power limitations, peak limiting techniques have been applied only to the input signal x by the transmitting device, e.g. the subscriber station 102 in FIG. 1. However, the channel estimator 114 uses a non-peak limited pilot sequence xp to estimate the channel matrix Ĥ. The resulting estimated channel matrix channel matrix Ĥ is, thus, based upon inaccurate data.